Inari sits at 69° north and divides cleanly into two very different seasons.
Winter (November–April) is the deep dark season — temperatures regularly below –20°C, the lake frozen solid, several centimetres of snow on the ground, and the Northern Lights overhead on clear nights. The polar night (kaamos) — when the sun does not rise at all — runs from late November into mid-January, though there are still a few hours of blue twilight even then. February to early April combines hard snow, increasing daylight, and the best chance of aurora. Pack seriously warm clothes; nothing British or Central European will do.
Summer (June–August) is the opposite: long warm days (the sun never sets between mid-May and late July), the lake open, the fells walkable, and the mosquitoes furious. This is the time for hiking, boat trips, and gold panning in Lemmenjoki.
Shoulder seasons (May, September, October) are quiet and atmospheric — autumn colours (ruska) peak in mid-September, the first snow falls in October. Few tourists either side of the busy aurora and Christmas season.